Showing papers in "Journal of the Institution of Electrical Engineers in 1954"

Discharge Inception and Extinction in Dielectric Voids

Abstract: Measurements of the voltage required to produce discharges in voids within a number of different dielectrics show that the discharge-inception voltage across the void is independent of the nature of the dielectric and of the void position, including the case in which the void is bounded by one of the electrodes, but depends upon void depth or thickness in the direction of the applied electric field. The measurements, including those made at pressures and temperatures other than n.t.p., are consistent with Paschen's law for metal-to-metal breakdown of gas.

Theory of Composite Guides: Stratified Guides for Surface Waves

Abstract: An analysis is given of surface-wave propagation along a plane interface separating air from a semi-infinite medium coated with several layers of different media. Formulae are derived for the general case of arbitrary media. Simple approximate expressions are obtained for the surface impedance of the guiding plane, the decay coefficient of the surface-wave field, the attenuation and phase-propagation coefficients: the use of the formulae is illustrated by the solution of a few particular problems. The application to single-wire transmission line is indicated.

The circuit development of the ampliphase broadcasting transmitter

Abstract: The paper describes the circuit design of a new type of broadcasting transmitter believed to be the most economical in equipment and valves and of the smallest dimensions and weight for a given power. It has an overall electrical efficiency and a modulation performance at least equal to that of the best known types. This latest type exploits phase-modulation internally in the transmitter to give a normal amplitude-modulated output?hence the term ?ampliphase system??amplitude-modulation derived from phase-modulation.

An experimental investigation of the properties of corrugated cylindrical surface waveguides

Abstract: The paper describes an experimental study of the properties of externally corrugated cylindrical metal surfaces when used for the support of electromagnetic waves. In particular brass rods of 3/16 in outside diameter and grooved transversely at regular intervals along the length were employed to guide cylindrical surface-waves of the kind familiar in single-wire transmission. Measurements were made at a number of frequencies in the range 2 500–10 000 Mc/s using a surface-wave resonator. In no case did the depth of corrugation exceed 0.050in.It is shown that a uniformly corrugated cylindrical surface acts as a satisfactory guide for a surface wave if there are at least two corrugations within the guide wavelength. The results of measurements on the surface reactance of corrugated guides are recorded in a number of graphs. An approximate expression for the surface reactance of a corrugated surface is derived in the paper and found to be in reasonable agreement with the experimental results, provided that the slot width is less than the slot depth, and each wavelength includes several corrugations.

The accuracy of the location of sources of atmospherics by radio direction-finding

Abstract: In recent years, increasing importance has been attached to the use of radio direction-finders for the location of thunderstorms. A large number of stations for this purpose are now in operation, and for the planning of further networks it is necessary to assess the accuracy to which storms may at present be located, and to delimit the areas in which location is satisfactory with the existing networks. An investigation of the accuracy of the United Kingdom network of twin-channel cathode-ray direction-finders, operating at frequencies near 10 kc/s, has shown that instrumental errors other than polarization errors are small, but that at particular stations, errors of several degrees have been caused by the hilly nature of the terrain and by buried cables. When the equipment is on a level site, free from obstructions, polarization errors are likely to be the main limitation to accuracy, particularly at distances of a few hundred kilometres and at night. Maximum errors of 2? or 3? may be expected in summer day-time and rather more in winter day-time. The magnitudes of polarization errors at night are not accurately known. Over a limited range of distance, around 400km, observations on continuous-wave stations exhibit, at times, r.m.s. errors of more than 20?, but there is evidence that errors of this magnitude do not normally occur with atmospherics. At distances greater than 1 000km, polarization errors may be expected to be no greater than 2? or 3?, even at night. At these distances there are also significant errors caused by interference between the atmospheric under observation and others arriving almost simultaneously. Although knowledge of the errors is far from complete, preliminary estimates of their magnitudes have been made and an estimate of the accuracy of storm location has been derived. It is not possible to express the potential accuracy in simple terms, since it depends not only on the bearing errors, the spacing of the stations and the distance of the storm, but also on the number of observations made and the manner in which they are interpreted. As an indication of the order of magnitude, it is estimated that with the present network and observation technique the probable error in position of a storm centre at a distance of 1 000km is about 20km by summer day, 50km by winter day and 100km by night. These preliminary estimates, particularly that for night-time conditions, must be regarded as approximate because knowledge of the bearing errors is still lacking in some important respects. In particular, a more complete knowledge is required of their amplitude distribution and of the degree of correlation, if any, between polarization errors at the different stations. The types of error discussed are liable to occur with all atmospherics direction-finders in current use; polarization errors, in particular, may be expected with any instrument with an aerial system incorporating either a single rotating loop or a pair of fixed crossed-loops.

The transient behaviour of ladder networks of the type representing transformer and machine windings

Abstract: The theory of the transient behaviour of certain ladder networks, having a finite number of sections, when subjected to a unit-function voltage surge is developed rigorously. The networks chosen are those usually employed to represent a coil or winding in which each section has inductance L; capacitance to earth Cg and capacitance between sections Cs. Solutions for the transient voltages produced in such a network when the end is open or short-circuited are obtained using the Laplace transform. The solutions are general and show that only certain frequencies are excited in such a network. In particular, the treatment indicates errors in the earlier work of Rudenberg which are important since they lead to inaccuracies concerning the stresses to be expected in a machine or transformer. Calculations show how the important ratio Cg/Cs affects the voltage distributions and stresses, and how special cases of engineering importance (transformer, machine, choke, artificial line) can be derived by selecting appropriate values of Cg/Cs. Finally, by allowing the number of sections to become infinite, solutions are obtained for continuous windings and for a transmission line.

Waveform responses of television links

Abstract: Referring particularly to point-to-point links, the paper is concerned with the long-outstanding problems of measuring and specifying the transmission performance of those parts of a television system which can be treated as linear networks. Such problems are more easily studied in terms of waveform responses than of steady-state characteristics, because the permissible distortions of television signals can be expressed more directly in terms of time than of frequency. Studies have been somewhat hampered by the dearth of published information on waveform responses taken under practical everyday conditions. The paper offers assistance by giving typical examples from a series of tests, made by the Post Office with the co-operation of the British Broadcasting Corporation, on some of the links and items of equipment used in the 405-line system. The responses were taken with the aid of a set of sine-squared pulse and other test signals shown by experience to be particularly convenient for the purpose. Two complementary methods of specification are proposed, one for rapid routine-testing purposes, and the other to meet the more precise requirements of acceptance-testing work. Both methods are framed in terms of a "rating factor," the value of which can be varied to provide any desired degree of stringency. The acceptance-test method takes advantage of the simplification afforded by the time-series method of expressing waveform responses.

The measurement and calculation of axial electromagnetic forces in concentric transformer windings

Abstract: An experimental method of determining the axial forces in power transformers is applied to a transformer constructed so that various tapping arrangements can be set up, and curves are given of the forces which occur with typical methods of tapping concentric windings. It is considered that the simple "residual ampere-turn" method of calculating axial forces is sufficiently accurate for design purposes, and a number of formulae are given which should cover most of the requirements of the transformer designer. The distribution of axial force around the periphery of the coils is investigated experimentally.

R.F. conductivity in copper at 8 mm wavelengths

Abstract: Measurements of r.f. conductivity in copper have been made at 8mm wavelengths. It is shown that, in addition to the known effect of surface roughness, the conductivity may be considerably reduced below the d.c. value by surface layers of low conductivity and stress in the bulk material. This can be overcome by etching and annealing, or by a process designed to cover the surface layers. Under suitable conditions the d.c. conductivity can be obtained.

The minimum-moment-of-error-squared criterion: a new performance criterion for servo mechanisms

Abstract: A technique is described and standard integral forms are presented for the application of a new performance criterion for servo mechanisms based upon minimizing the time moment of error-squared of a system in response to a given signal. Examples of its use are given, in the course of which a novel graphical method of analysing servo mechanisms is described. The new criterion is a comprehensive one depending upon the complete error transient. It is related to the minimum-integral-of-error-squared criterion, which has been previously proposed. The latter criterion, while convenient for mathematical manipulation, suffers from the severe disadvantage that systems based upon it do not give the best practical results. As a consequence of considerable practical experience with the simpler types of servo mechanism it is usual to express empirically design requirements which lead to systems yielding the best practical results. Any criterion of practical significance should satisfy these requirements in such cases. It is the purpose of the paper to show that the minimum-moment-of-error-squared criterion does satisfy these requirements although the minimum-integral-of-error-squared criterion does not, and to recommend that the time-weighted criterion be adopted for the design of the modern special-purpose system, where the original empirical requirements are not necessarily appropriate.

Noise in silicon microwave diodes

Abstract: The various kinds of noise fluctuation which occur in silicon microwave diodes are discussed in the light of measurements at widely differing frequencies of the noise produced when the diodes are excited by d.c. bias. It is found that flicker noise which is dominant at 1 600 c/s may extend to 45 Mc/s and beyond. By selecting diodes for low noise output and restricting the measurements to low bias currents, good agreement is found at 45 Mc/s with the calculated values of shot noise; it is established, in particular, that the thermal noise which occurs for zero bias may be calculated in terms of the shot-noise concept. In the region of 30 000-40 000 Mc/s the noise produced by reverse bias is probably attributable entirely to shot noise. Finally the bearing of these measurements on the problem of noise during mixer operation is discussed briefly.

An 8-MeV linear accelerator for X-ray therapy

Abstract: The paper gives a general account of a 4-year research and development programme which has resulted in the installation at Hammersmith Hospital of a new type of super-voltage therapy equipment. This machine produces 8-MV X-rays, and stabilized outputs of up to 200 r/min at 1 m are available after filtration. Acceleration of the electrons is obtained by a 3-m length of corrugated waveguide energized by a magnetron giving a pulsed output of approximately 2 MW peak power at a free-space wavelength of 10 cm. Since the accelerating unit is fixed with its axis horizontal, the electron beam is deflected through a right angle before reaching the target, and the deflector system is made rotatable, to give variable direction of the X-ray beam. Many clinical facilities are available, and, in addition, the electron beam may easily be extracted.Early Sections of the paper deal with the background to the programme, the underlying principles and the basic design. These are followed by a description of the equipment and some details of its construction. The investigation of the performance is covered in some detail, and certain improvements in the understanding of the properties of the accelerating waveguide are discussed. Finally, a comparison is made between measured and calculated performance and the agreement is shown to be satisfactory.

Some factors affecting the measurement of the electric strength of organic liquids

Abstract: Some time ago a research programme on the electric strength of pure liquids was started and has led already to a number of publications dealing with the strength of simple organic liquids. In the paper are given the results of a systematic investigation of some of the extraneous effects associated with the method of measurement. The tests were made under carefully controlled conditions with the test cell incorporated in a closed distillation system. Among the factors influencing the electric strength of pure liquids, the effects of moisture and gas in the liquid, as well as of gas adsorbed on the electrode surface, were examined. It was found that, for pure liquids, the conditioning of electrodes, i.e. the gradual increase of the electric strength with successive voltage applications, appears to be due to a gradual degassing of the electrodes. No such effect was observed with artificially degassed electrodes. Experiments on the effect of the electrode geometry have shown the remarkable result that, using a test cell of sufficient diameter, the electric strength of pure liquids does not depend on the size of the electrodes and their spacing, except for very small spacings, where there appears to be a tendency for the electric strength to increase.

Distributed amplifiers: some new methods for controlling gain/frequency and transient responses of amplifiers having moderate bandwidths

Abstract: The limitation (first shown by Wheeler) of the gain-bandwidth product of an amplifier stage can be overcome, as earlier proposed by Percival, by connecting valves so that their input and output capacitances form the shunt elements of low-pass ladder networks. The resulting distributed amplifier does not give constant gain or delay within the pass band if simple networks are used. Improved constancy of gain can be obtained without using dissipative networks, however, or relying on the input conductance of the valves. Dissipative networks can also give improved phase/frequency response, and a gain/frequency response approaching Gaussian within the useful portion of the pass band. Amplifiers using dissipative networks are reasonably simple to construct, but suffer from the limitation that the gain-bandwidth product increases only as the square root of the number of valves in the stage, making amplifiers with more than about five valves of limited advantage. The practical realization of inductors reasonably free from transmission-line effects at high frequencies has presented difficulties, which were solved by the use of air-cored solenoids of very small physical size.

Service experience of the effect of corrosion on steel-cored-aluminium overhead-line conductors

Abstract: The paper describes the experiments carried out during the last 20 years on the corrosion of steel-cored-aluminium overhead-line conductors on the Grid system. It is found that the average rate of deterioration, based on the reduction in mechanical strength, is 0.4?0.5% per year, and that the average service life of the conductor is 25?30 years. The rate of deterioration varies greatly in different localities, extreme values for the life of the conductor being from 3 to, say, 50 years. The mechanism of corrosion is described, and it is shown that there are two types of corrosion, one being associated with industrial districts and the other usually with coastal regions. The prevention of corrosion is discussed and information is given on service experience with greases and other protectives. Recommendations are made on methods of protecting conductors by treatment with a suitable grease.

Harmonic Distortion of Frequency-Modulated Waves by Linear Networks

Abstract: An expression is obtained connecting the distortion of the modulating signal with the distortion of the r.f. spectrum when an f.m. carrier is passed through a linear network. It is used to obtain the harmonic distortion when the signal is a single tone. In the derivation of the expression it is assumed that departures of the phase characteristic of the network from linearity with respect to frequency, and of the amplitude characteristic from constancy, are small. These assumptions are made on the ground that we are usually interested only in networks that produce small distortion of the modulation. However, it is shown that such small distortion is more generally produced by an amplitude characteristic which is sinusoidal with respect to frequency, provided that the distortion due to the phase characteristic is small. The theory could be extended straightforwardly to cover the case of small deviations from such sinusoidal characteristics, although the extension is not made in the paper. Tables of functions involved in calculating distortion by the present approach are provided, and comparison is made with the result obtained by Van der Pol using Carson and Fry's operational method, and with some measurements.

The recovery of electric strength of an arc-discharge column following rapid interruption of the current

Abstract: A new technique for investigating the recovery period is described. A rectangular pulse of arc current is used and a probing voltage impulse is applied to the decaying arc channel at varied delay times after current interruption. The method has certain advantages over the normal a.c. method of testing. Reignition-voltage/time curves are presented for different conditions of electrode material, gas, gap length, arc current, arc duration and reignition polarity, with gas pressures at or near atmospheric. Observations are also made on the dynamic characteristics of the electrical discharge following reignition. Polished-copper and polished-tungsten electrodes require a reignition voltage of 355 volts within 3 microsec after current interruption for arc currents of 20 amp and less. Pipped-copper, pipped-tungsten, mild-steel, aluminium and graphite electrodes show continuing emission of electrons after current interruption to a varying degree, with corresponding reduction of the reignition voltage. The reignition phenomena are markedly affected by the polarity of the reignition. The arc anode normally attains a higher temperature than the cathode, so that continuing thermionic emission of electrons is more prevalent. Thus reignitions of reverse polarity from the original arc discharge tend to be lower than those of the same polarity. Different gases recover electric strength in the order: sulphur hexafluoride, carbon dioxide, oxygen, air, nitrogen and argon.

Standard frequency transmissions

Abstract: Standard frequency transmissions were first used in 1923, and since then their importance has steadily increased. It is intended to give them a world-wide continuous coverage, and the frequencies of 2.5, 5, 10, 15 and 25 Mc/s have been allocated to this service. Transmissions from the United Kingdom have been resumed and extended since the end of the 1939-45 War and give good coverage over the important area within a radius of about 1000 miles, in which reception of the United States WWV service has been poor, particularly during the last few years. The accuracy required in the measurement of frequency can for many purposes be expressed in cycles per second rather than as a proportion of the frequency, and it has in consequence increased as the higher-frequency radio bands have come into use. A close study of the accuracy that can be achieved has therefore been made at the National Physical Laboratory. It has been found that, although an accuracy of frequency comparison of ±2 parts in 108 can be relied upon when conditions are such that the wave is reflected from the E-layer of the ionosphere, random variations of several parts in 107 occur at other times, in addition to a systematic effect at dawn and sunset. An accuracy of ±2 parts in 109 can, however, be achieved with the experimental transmission on 60 kc/s. Accuracies of ±2 parts in 109 can also be achieved by using the timing pulses of the high-frequency transmissions, but this method involves the use of high-grade local standards and fairly elaborate auxiliary equipment.

A speech-waveform correlator with magnetic-tape delay and electronic multiplication

Abstract: The paper describes an instrument designed to compute the auto- and cross-correlation functions of speech sounds having a duration not exceeding 10 sec.The instrument comprises a twin-channel magnetic-tape delay and storage system, an electronic computor and certain ancillary apparatus. Analogue representation is used throughout, and the computed function is plotted automatically by a recording milliammeter.By recording at tape speeds faster or slower than normal, the operating range of 200–20000 c/s can be scaled up or down as required. It follows, therefore, that the instrument may be of use in a number of applications other than speech analysis.Details are given concerning the design of the instrument, its operation, and the accuracy of computation. In general, an accuracy of ±2% or better can be replied upon.A few recorded correlograms are given, and a table of correlation transforms is appended for use in testing the instrument.

An apparatus for the detection of weak ionospheric echoes

Abstract: An apparatus is described with which weak ionospheric echoes can be detected and their heights measured; it makes use of frequency modulation and coherent integration. With a transmitted power of 250 watts and an output bandwidth of 1 c/s the apparatus makes possible the detection of echoes corresponding to a reflection coefficient of 3×10−5 (attenuation of 10 nepers). It is pointed out that the principle might be of use in the design of sensitive radar equipment.

A short modern review of fundamental electromagnetic theory

Abstract: The paper is concerned with the teaching of the fundamentals of electromagnetism; it is divided into three parts. Part 1 deals with a method of presenting electromagnetic concepts to the student which takes the inverse square law of force between magnetic poles or electric charges as its starting points. This method is considered to result in a more satisfying sequence of thought than that usually adopted, because the symmetry of magnetic and electrical relations is stressed. Part 2 is a detailed discussion of the teaching of Faraday's laws of electromagnetic induction. Part 3 is a presentation of Maxwell's equations in terms of electric and magnetic flux density.

The determination of optimum process-controller settings and their confirmation by means of an electronic simulator

Abstract: Simplifying assumptions regarding the nature of certain types of continuous processes lead to transfer functions which are amenable to analysis. Criteria for optimum control based on the transient responses of such systems when under proportional plus derivative plus integral control are given.The effects of integral control at low frequencies are studied, and it is shown how conditional stability can be avoided when the process contains an integration. A low-frequency damped oscillation which can occur with three-term control is investigated, and the condition for its elimination is derived.An analytical method is developed which, for any given integral control setting, makes it possible to determine the optimum values for the two remaining controller settings. It is shown that the introduction of integral control allows an increase in the proportional control factor for the same degree of damping of the first normal mode of oscillation.Experimental results are given showing how an electronic simulator for process control has been used to confirm the results of the analyses and to assist in determining the optimum settings for integral control.For completeness, a brief description of an automatic-process-control system with definitions of the terms used is included in the paper.

The transistor regenerative amplifier as a computer element

Abstract: A method of designing transistor 2-state circuits which accepts transistors having a relatively large spread in characteristics has already been described. Although subsequent improvement in the uniformity of transistors has removed the primary object of the system, it has been retained in the circuits described in the paper because of the many other advantages it confers where accuracy and speed of operation are important. The transistor is considered as a regenerative amplifier, and the first part of the paper describes various methods of connection and endeavours to evaluate the merits of each circuit. The second part illustrates the application of some of the basic circuits to more complex circuits of the type used in digital computers. These circuits operate at a basic frequency of 125 kc/s and use rectangular pulses to allow adequate time for comparing two or more waveforms and performing operations on them. It is concluded that, owing to its adequate stage gain, low output impedance and standard output pulse, the transistor regenerative amplifier constitutes an economical and versatile computer element.

Determination of the static and dynamic elastic properties of resilient materials

Abstract: The investigation was carried out to satisfy the need for reliable information on resilient materials used for reducing the transmission of structure-borne noise. Two machines were designed for the dynamic testing of twin samples of material under known compressive loads, and their development and performance are described. Results on typical materials have been obtained over a range of frequencies from 1 cycle in 10 minutes to values in the lower audio register. The lower limiting value of dynamic stiffness has been called the "incremental stiffness," and the method of measuring it is described. The effects of wave motion in soft rubber have been investigated for two small samples using a high-frequency apparatus; and at certain frequencies, depending on the dimensions of the specimen, the transmitted force for a given oscillatory deformation is shown to be about seven times that at low frequencies. By using simple beam arrangements, measurements of both creep and incremental stiffness have been made on three types of material, over a period of about 1000 hours of continuous loading. Results are presented showing that most creep occurs during the first 100 hours. The techniques described enable useful information on the performance of resilient materials, as used for the reduction of vibration, to be obtained.

Submerged telephone repeaters for shallow water

Abstract: In 1943 the British Post Office laid the first submerged repeater in a working cable system, and since that date the technique has been developed so that such units are now being used freely in the shallow waters of the Continental Shelf.Shallow-water repeaters are of particular interest to the Post Office, with its extensive system of submarine cables to the Continent. While the interest in repeaters suitable for transoceanic routes in deep water remains and increases, independent solution of the lesser problem has been well worth while and has provided valuable background and data for the solution of the greater problem.The Post Office has standardized one system which provides 60 telephone circuits of C.C.I.F. standard over a single cable with up to at least 10 repeaters in tandem. Systems transmitting 36 circuits have also been designed by a British contractor, and other arrangements are practicable if required. However, the advantages of standardization of repeaters and terminal equipment are obvious.The paper discusses the problems involved and the methods adopted or proposed for their solution. At present, 18 cables have been equipped with repeaters or will be so equipped in the near future; 47 repeaters are involved, the maximum number in one cable being seven.

Analysis of Hunting Phenomena in Power Systems by Means of Electrical Analogues

Abstract: The paper presents an elementary approach to the theoretical considerations upon which electrical analogues can be used to simulate hunting phenomena in power systems. The major assumption is that in the case of a deviation of the pole axis of the rotor of a machine from its normal steady-state condition, the amplitude of such angular displacement must be small. As an example, a simple electrical analogue is set up to simulate a given electro-mechanical system. The frequency and amplitude of natural oscillations determined experimentally on the analogue agree very well with the results of a mathematical analysis.In order to extend the scope to system stability problems, a 10-kc/s network analyser utilizing analogue-computer technique is described. The main object is to produce a unit capable not only of solving steady-state stability problems, but also of producing directly the system swing curves for transient problems, without the necessity for step-by-step computations, as are required with the usual a.c. network analyser. Further work on this analyser is being carried out and will be reported in a future paper.

Ionospheric absorption at vertical and oblique incidence

Abstract: Measurements of the ionospheric absorption of radio waves of frequency 9.15 Mc/s over an 800-km path, and of frequency 5.455 Mc/s at vertical incidence at the mid-point of this path, have been made simultaneously over a period of 14 months. A discrepancy between the observed results and those predicted by Martyn's absorption theorem is found, which varies diurnally. The oblique-incidence results are examined in detail, and empirical formulae for diurnal and seasonal variation of absorption are derived.

A technique for investigating the electrical properties of oil-impregnated paper

Abstract: A description is given of a technique which has been developed for the investigation of the electrical properties of oil-impregnated paper, with special reference to the type of lapped-paper dielectric used in power cables.The equipment described is versatile, permitting a wide range of electrical tests to be made on one type of specimen and enabling the effect of variations in construction or component dielectric materials to be assessed rapidly and conveniently. The system is well suited for routine quality control as well as for research or development.

Some notes on the absorption of radio waves reflected from the ionosphere at oblique incidence

Abstract: The results of some oblique-incidence measurements on the ionospheric absorption of radio waves are considered. For both short and very long distances of transmission it is found that there is a marked discrepancy between the observed values and the values calculated from simple theoretical formulae. The contribution of multiple ionospheric reflections to the intensity of the received signal is briefly discussed.